3 Phase Motor Blowing Fuse

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Saturn_Europa

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Fishing Industry
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Electrician Limited License NC
I have a 15 hp, 3 phase, 480 v, 17 FLA, motor, running a multistage centrifugal pump. It was taken out of service today so our mechanics could replace the upper and lower bearings.

After re-installation it has been non-functional. The motor will come up to speed, run for a few minutes, pump water then trip the overloads. It has been blowing a single fuse, single phasing then dropping out on overloads. The fuses are LPJ-30SP , it was fully functional before the mechanics pulled it. It just had bad bearings.

The first time it did this it blew the B phase fuse. The second time it blew the C phase fuse.

I megged out the wire going to the motor from the motor starter and it was good. I megged the windings to ground and they are good. I ohmed out the windings and have roughly 0.9 ohms on each winding. There is no problem with the motor that I can detect.

The pump and motor spin freely by turning the fan on the top of the motor.

I am leaning to a phase to phase short caused by damage during bearing replacement. But I want to make sure I have thought about everything before I tell the guys they have to pull it again in 90 degree heat. Its not a fun job.

But I think I have checked everything, I can with the meters that I have. The wires are good going to the motor. Windings arent shorted or open. 480/277 V on the top of the motor starter before it I start the motor. Everything looks ok.

Any ideas?
 
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Jraef

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Have you monitored the current?

If it runs for a while and then blows the fuse, that points first to it being an overload, but the OL relay is not tripping for some reason. But the other possibilities are that the control signal is "bouncing" and making it cycle on and off too fast, water is getting into the pump after it runs and develops pressure for a while, or something is shorting out when it heats up.

If absolutely nothing else has changed, I would gravitate toward water getting into something over time.

A more remote possibility is that there is a thrust bearing on the pump that is seizing up. You would not be able to see that by spinning it by hand.
 

JFletcher

Senior Member
Location
Williamsburg, VA
How are the bearings lubricated? Oil cups? Perhaps the bearings are underlubricated or tight, causing heat, drag, overamperage, blown fuse. You work at a WW plant, right? I watched our 150 HP furnace ID fan weld itself together after a bearing and oiler replacement. cause: a small plug of grease in the new outboard bearing oil cup blocked oil flow.

Second thought: what media is the pump trying to move?
 

Saturn_Europa

Senior Member
Location
Fishing Industry
Occupation
Electrician Limited License NC
Have you monitored the current?

If it runs for a while and then blows the fuse, that points first to it being an overload, but the OL relay is not tripping for some reason. But the other possibilities are that the control signal is "bouncing" and making it cycle on and off too fast, water is getting into the pump after it runs and develops pressure for a while, or something is shorting out when it heats up.

If absolutely nothing else has changed, I would gravitate toward water getting into something over time.

A more remote possibility is that there is a thrust bearing on the pump that is seizing up. You would not be able to see that by spinning it by hand.


It blows the fuse on start up then trips the overloads after a minute or two of running on only two phases.
 

Saturn_Europa

Senior Member
Location
Fishing Industry
Occupation
Electrician Limited License NC
How are the bearings lubricated? Oil cups? Perhaps the bearings are underlubricated or tight, causing heat, drag, overamperage, blown fuse. You work at a WW plant, right? I watched our 150 HP furnace ID fan weld itself together after a bearing and oiler replacement. cause: a small plug of grease in the new outboard bearing oil cup blocked oil flow.

I thought it might be something like that too. But everything turns very freely by hand.

They are standard shielded ball bears greased with a extreme pressure grease.
 

Saturn_Europa

Senior Member
Location
Fishing Industry
Occupation
Electrician Limited License NC
Wrong size or type fuse if it blows on startup?


The equipment has been in service for 18 months and its only blowing one fuse out of three, so IDK. Using NEC chapter 430, I could go up to 40 amps. But I am hesitant to do that because the equipment has been in service for so long.
 

kwired

Electron manager
Location
NE Nebraska
It blows the fuse on start up then trips the overloads after a minute or two of running on only two phases.
Tripping overload after running on just two lines would be expected, so you need to narrow down why it is blowing fuses.

One possibility - metal parts/tools got left in the motor when it was being worked on and were pulled into the coils when they were energized causing a turn to turn fault or even turn to ground fault.
 

Saturn_Europa

Senior Member
Location
Fishing Industry
Occupation
Electrician Limited License NC
How are the bearings lubricated? Oil cups? Perhaps the bearings are underlubricated or tight, causing heat, drag, overamperage, blown fuse. You work at a WW plant, right? I watched our 150 HP furnace ID fan weld itself together after a bearing and oiler replacement. cause: a small plug of grease in the new outboard bearing oil cup blocked oil flow.

Second thought: what media is the pump trying to move?


Yes, sir!! Still living the dream treating wastewater right by a nuclear power facility :)

Its pumping through spray nozzles that backwash disc filters.
 

JFletcher

Senior Member
Location
Williamsburg, VA
Yes, sir!! Still living the dream treating wastewater right by a nuclear power facility :)

Its pumping through spray nozzles that backwash disc filters.

:cool:

So no chance the impeller is ragged up. I take your first post to mean the motor bearings were replaced, and the pump was not disassembled, yes?

Im still leaning toward a mechanical problem. Are all the isolation valves back in the proper positions? And as was mentioned, rotation is correct?

Were/ are you able to check amperage on startup and right before trip?

Dumb question, but did maintenance rebuild that motor and put it back in, or swap for another? Any chance they grabbed a 20 or 25hp motor instead of a 15?
 

Saturn_Europa

Senior Member
Location
Fishing Industry
Occupation
Electrician Limited License NC
:cool:

So no chance the impeller is ragged up. I take your first post to mean the motor bearings were replaced, and the pump was not disassembled, yes?

Im still leaning toward a mechanical problem. Are all the isolation valves back in the proper positions? And as was mentioned, rotation is correct?

Were/ are you able to check amperage on startup and right before trip?

Dumb question, but did maintenance rebuild that motor and put it back in, or swap for another? Any chance they grabbed a 20 or 25hp motor instead of a 15?


Yep, they rebuilt it and put the same motor back. I'll double check for rags tomorrow. Rotation is correct. It was right at the end of the day and I didnt really check amps. I was readying voltages and saw the B phase go to zero within 30 seconds of motor starting. It ran for another minute or two and went out on overload.

I'll let you know what I find tomorrow.
 

GoldDigger

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Retired PV System Designer
Yep, they rebuilt it and put the same motor back. I'll double check for rags tomorrow. Rotation is correct. It was right at the end of the day and I didnt really check amps. I was readying voltages and saw the B phase go to zero within 30 seconds of motor starting. It ran for another minute or two and went out on overload.

I'll let you know what I find tomorrow.

So what was the line current during the time after the starting surge and before the fuse opened?

Also is there any chance that there was a restriction in the flow (restrictor plate, valve, nozzle setting, etc.) which was removed as part of the replacement process? If the pump is moving more water it well consume more power.
 

Saturn_Europa

Senior Member
Location
Fishing Industry
Occupation
Electrician Limited License NC
Tripping overload after running on just two lines would be expected, so you need to narrow down why it is blowing fuses.

One possibility - metal parts/tools got left in the motor when it was being worked on and were pulled into the coils when they were energized causing a turn to turn fault or even turn to ground fault.


I agree. It blows right at start up, and the fuses arent cheap so I am brainstorming ideas to check tomorrow. I will probably pull amps at start up tomorrow, but if the fuse blows, amp readings with a line out wont be much help.
 

Saturn_Europa

Senior Member
Location
Fishing Industry
Occupation
Electrician Limited License NC
So what was the line current during the time after the starting surge and before the fuse opened?

Also is there any chance that there was a restriction in the flow (restrictor plate, valve, nozzle setting, etc.) which was removed as part of the replacement process? If the pump is moving more water it well consume more power.


I'll have to check first thing tomorrow. I'll borrow a few clamp on amp meters and read all legs during start up.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
... The motor will come up to speed, run for a few minutes, pump water then trip the overloads. It has been blowing a single fuse, single phasing then dropping out on overloads. ...

It blows the fuse on start up then trips the overloads after a minute or two of running on only two phases.

... I was readying voltages and saw the B phase go to zero within 30 seconds of motor starting. It ran for another minute or two and went out on overload.

I'll let you know what I find tomorrow.

... It blows right at start up, and the fuses arent cheap so I am brainstorming ideas to check tomorrow. ....

In the world of fuse clearing, there can be a big difference between "a few minutes", "on start up", "within 30 seconds" or "right at start up". I know I'm sounding persnickety here, but you have to realize the time frame is important.

So taking the voltage going to zero as the clearing of the fuse, 30 seconds is actually a long time even for a time delay fuse; on that specific fuse the time delay is 10 seconds at 500% rated, so in your case that's 150A. Short Circuit current would likely be up to 17-20x FLC, so in the 280-340A range, which means that fuse would clear a LOT faster than 30 seconds.

On a motor rated 17A FLC, Locked Rotor Current will only be approx. 102A. So it's possible that the shaft is locking and because the LRC is a lot lower than the 500% rating of the fuse, it's taking 30 seconds to clear rather than 10 seconds.

To me then it's looking more like a locked rotor situation. Again, if it was a rag or something in the impeller, it would not turn freely by hand either. Since it only happens AFTER you start it, and only clears the fuse 30 seconds later, it's something that changes on startup. I'm still thinking a bad thrust bearing. In a vertical turbine, the thrust bearing is there for when the turbines cause uplift in the pump shaft, so when that happens, the thrust bearing allows the shaft to spin against it. If that bearing is seized or jammed down too tight, as soon as the shaft lifts, it locks up. That would only happen AFTER the pump starts to pump.

Taken from this article for illumination:

Start-Up Considerations
When a pump is first turned on, the pump usually operates at very high capacities, because the motor gets up to speed I just a few seconds, and it may take somewhat longer for the head to build up. The pump is therefore likely to operate in the very high capacity range where upthrust occurs. In most installation, the head builds up almost immediately, so that the upthrust is only “momentary.” However, even though it is momentary, it is present, and the equipment must be designed to take this upthrust. If there are hundreds of feet of lineshaft, and the head equals the setting, the dead weight of the shaft will normally “absorb” the upthrust. If the unit is “close-coupled,” or if it is a submersible, then the upthrust load will be transmitted to the driver and adequate provision must be made.

thrustbrg2.jpg

So it's possible that when the pump was "rebuilt", someone did something wrong with the thrust bearing and it's either missing now, or they misinterpreted what it was and why there appeared to be "play" in it, so they tightened it down tight or something. I say that because I've seen it happen on more than one occasion at motor shops that were unfamiliar with vertical turbine pumps.
 

Saturn_Europa

Senior Member
Location
Fishing Industry
Occupation
Electrician Limited License NC
In the world of fuse clearing, there can be a big difference between "a few minutes", "on start up", "within 30 seconds" or "right at start up". I know I'm sounding persnickety here, but you have to realize the time frame is important.

So taking the voltage going to zero as the clearing of the fuse, 30 seconds is actually a long time even for a time delay fuse; on that specific fuse the time delay is 10 seconds at 500% rated, so in your case that's 150A. Short Circuit current would likely be up to 17-20x FLC, so in the 280-340A range, which means that fuse would clear a LOT faster than 30 seconds.

On a motor rated 17A FLC, Locked Rotor Current will only be approx. 102A. So it's possible that the shaft is locking and because the LRC is a lot lower than the 500% rating of the fuse, it's taking 30 seconds to clear rather than 10 seconds.

To me then it's looking more like a locked rotor situation. Again, if it was a rag or something in the impeller, it would not turn freely by hand either. Since it only happens AFTER you start it, and only clears the fuse 30 seconds later, it's something that changes on startup. I'm still thinking a bad thrust bearing. In a vertical turbine, the thrust bearing is there for when the turbines cause uplift in the pump shaft, so when that happens, the thrust bearing allows the shaft to spin against it. If that bearing is seized or jammed down too tight, as soon as the shaft lifts, it locks up. That would only happen AFTER the pump starts to pump.

Taken from this article for illumination:



View attachment 15155

So it's possible that when the pump was "rebuilt", someone did something wrong with the thrust bearing and it's either missing now, or they misinterpreted what it was and why there appeared to be "play" in it, so they tightened it down tight or something. I say that because I've seen it happen on more than one occasion at motor shops that were unfamiliar with vertical turbine pumps.


Thank you for the reply Jraef, thats definitely something to look into. I'll let y'all know what I find tomorrow.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
...

To me then it's looking more like a locked rotor situation. Again, if it was a rag or something in the impeller, it would not turn freely by hand either. Since it only happens AFTER you start it, and only clears the fuse 30 seconds later, it's something that changes on startup. I'm still thinking a bad thrust bearing. In a vertical turbine, the thrust bearing is there for when the turbines cause uplift in the pump shaft, so when that happens, the thrust bearing allows the shaft to spin against it. If that bearing is seized or jammed down too tight, as soon as the shaft lifts, it locks up. That would only happen AFTER the pump starts to pump.
Since the OP says that it pumped water for awhile (but not whether it was at full flow) the "locked" rotor is probably just a lot of extra drag.

What is confusing me is that after one fuse blows the current in the remaining two fuses is not high enough to blow one of them (and the pump continues to move water?).
My brain is not coming up with anything that would cause a momentary or short term overcurrent but allow the pump to continue running for minutes until the overload trips.
Maybe something about the torque/thrust that is reduced when the first fuse blows, reducing the resistance to rotation, hence reducing the current seen by the remaining pair of fuses?
 
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